Process for the production of molybdenum trioxide and tungsten trioxide



1927. i, May' 17 P. scHwARzKoPF PRocEss Fo THE PRODUCTION 'of' MoLYBDENUNTRIoXfIDE AND TUNGSTEN TnIoxIDE Filed -July 2s. lsesl Patented VMaylv?, 1927.-.. 'i

PATEN PAUL scnwnnzxorr, or :Baum-CHARLOTTENBUB'G, annum.

rnocnss non Tm: rnonUcTIoN or moLYnnENUM Tnroxmn Nn TUNGSTEN TnIoxInE. v

Application' mainly as, 192e, serial No. 124,359, and in Germany apra as, 192s.'

The invention relates to a process for the production of molybdenum trioxid'e l(Moos) and tungsten trioxide (WOS) in an extremely pure state directly from the ore, and also tov molybdenum, who'le scheelite (CaWO4) and wolframite (FeWO4') contain tungsten. The usual method adopted in Working up these minerals is to subject the ores which have Gul been refined and concentrated as muchy as possible by mechanical means, either alone or with suitable additions, to an annealing process in order to convert the; tungsten or molybdenum in the ores into ammonium or alkali-tungstate or molybdate, which is solubleiny water, from which after suitable purification the desired tungsten trioxide or molybdenum trioxide -is'- obtained posing with mineral acids. For example, molybdenum trioxide is .obtained from'molybdenite by the following method. After the ores have been refined and concentrated, the molybdenum sulphide in the same is convertedv by roasting into molybdenum trioxide and the latterl is dissolved out of the roasted mass by ammonia.

'lhe molybdenum trioxide is then extracted from thef ammoniacal solution either by precipitating with mineral acids and subsequent calcining or by decomposing thcrystallized and recrystallized ammonium molybdate by' r a direct calcining process. Wulfenite is converted, after concentration, into molybdenum sulphide by melting with alkali carbonates, sulphur and carbon, this sulphide being also roasted and further treated as described in connection with the molybdenite.

In the production of tungsten trioxide, for example from Wolframite, a disintegrating rocess is carried out. by calcining with soda, iixiviating with water, then purifying the resulting sodium tungstate solution, precipi-v tating the tungsten trioxide hydrate from the hot solution with nitric acid and converting the hydrate into WO3 by calcination.

Irrespective of whether the molybdenum Atrioxide or tungsten t-rioxide is obtained in one of the methods described or byV any other known chemical means, it is always Acentage of hundredt s.

produced linally from ,alkali or ammonium molybdates or tungstates, into which the molybdenum or tungsten in the ore had first .of all to be converted,as`these compounds can be easily recrystallized and therefore afford' the greatest security that the products ,will be obtained in a very pure form. However, the oxldes obtained are never abso lutely pure, although they are placed on the market' as chemically pure. In addition to ammonia or :alkali they also contain small quantities, or traces at least, of iron and otherlmpurltles.

It is a fact that even the slightest trace of impurities may be detrimental when the molybdenum trioxideand tungsten trioxide are used for certain purposes,` for instance when they are Worked up into wires' and sheet metal for incandescent lamps, Rntgen tubes and electron tubes, and this detrimental effect would be evident even if the impurities in t-he acid onl amounted to a per Moreover, all the methods of production carriedout in practice toda are relatively very expensive. 'F or examp e, the cost of extractlng molybdenum trioxide (M005) amounts, according tothe method applied, to about 6080% of the value of the MOO3 in the concentrated ore.

Therefore, attempts havebeen made for some time toobtain these by a simple processdirect from the ore. Endeavor-s of this kind are known more especially for obtaining molybdenum trioxide.

'Whler has already the Annalen der Chemie und Phar1nacie, a process for obtaining molybdenum trioxide from the natural molybdenum sulphide, conmeans, Valthough Whlers suggestion Was taken up by the most famous chemists who succeeded him. For instance, Debray points out in 1868 that it would be ofgreat advantage for determining the atomic weight described in 1856, ili` of molybdenum, if it Werecpossible to make ues of VVhlers suggestion for producing absolutely pure molybdenum trioxide. However, in repeatiiio` lVhlers experiments he finds that the su limated oxide at its teinperature of condensation, attacks glass and thus always contains silica. Debray therefore uses a platinum tube instead of the glass tubeemployed b v Whler, but then obtains a molybdenum trioxide which has such a small density and consequently such a large volume, that Debray was obliged to dissolve it again in ammonia, in orderto concentrate it. However, this method introduces another impuritv into the product, which can only be purified with great difiiculty on a small scale in the laboratory, so that Debrays process is impracticable forthe technical production of molybdenum trioxide on a large scale. This is the reason why Moisson still says in 1905 that the molybdenum anhydride obtained according to Whlers process is not ure; p It may be mentioned here that similarly to the practice adopted commercially, tung sten trioxide vand molybdenum trioxide are understood in this case as referring to WO3 and M003. Theoretical chemistry refers to these compounds as acid anhydrides.

Finally, Whlers suggestion for obtaining molybdenum trioxide is taken as basis in. the U. S. Patent'1, ll8,150 of 1914,Which y proposes to pulverize molybdenitel and heat it at temperatures above 790 C. in an at-L mosphere which contains oxygen .in excess; the sublimated molybdenumitrioxide is collected in bag filters. However, in order toy be able to sublimate molybdenum trioxide in this manner, it is not possible to exceed a temperature ofi800 C. for reasons which Will be reverted to later, In view of the extreme importance of obtaining the inolybdenum trioxide in the purest form for the branches 'of industry previously referred to, this process4 has also been observed and tested. It has been found, however, as Ullmann says in his Enzyclopdie der technischen Chemie, vol. 8, page 186, that it is very difiicult to carry out this process in practice, and moreover the resulting molybdenum trioxide has the disadvantage which had previously been discovered by Debray, that it is not sufficiently dense and cannot thereforebe used for the production of incandescent filaments. Moreover, the mblybdenum trioxide obtained by this last method is not as pure as was expected. In the first place, it is stated in the said patent itself that the molybdenum trioxide obtained is Very pure but can be fui-'ther purified by similar resublimation by repeating the process.

Applicant has been able to discover the reasons Why lVhlers suggestion of years ago, although it has been repeatedly taken up by those engaged in the art, has led to no result and no method has been formulated which Will assure extremely pure molybdenum trioxide being produced on a commercial scale.' Whler and his successors all pass a current of air, in some cases with an excess of oxygen, over the finely divided ore during roasting. In such a case it is impossible to avoid fine particles of the inolybdenite, some in the roasted state and some not yet roasted, being carried along with the sublimated molybdenum trioxide and conveyed into the condensing chamber or the like, in which these impurities settle together with the oxide. This action is further assisted inthe cases in which oxygen is employed, by the fact that the loxygen supplied causes an explosive-like combustion of the molybdenite to take place, whereby very fine particles are broken off and mayjbe carried along for some distance by the stream 'of air. If it is taken into consideration that 0.01% ferrie oxide, that is to say'10 grs. ferrie oxide to 100 kgs.

molybdenum trioxide or\tungsten trioxide, i

apparatus, ventilators, pipe conduits and the like, cause foreign substances to be introduced into the oxide. Another reason for these`impurities is that already discovered by Debray, viz, that the subliniated molybdenum trioxide is rendered impure by the materials of the 'furnace and filter chamber.

My linvention obviates'the drawbacks of the processes referred to :ind makes it possible for the molybdenum trioxide to be produced directly from the `ore on a large scale in the factory. It is at least as simple .as the Process described in the U. S. patent and produces the purest tungsten trioxide and molybdenum trioxide at minimum cost. The resulting'oxides are sufiiciently dense, so that the tungsten trioxide can be Worked up Without any difficulty for the production of filaments for electric incandescent lamps, Rntgen tubes, electron. tubes and the like and is in a purer state than has ever been lobtained on a large scaleor scarcely even in small quantities iii laboratories. The resulting molybdenum trioxide, for example dissolves in ammonia, forming an absolutely clear solution which contains no impurities i of sulphurous or sulphuric acid and-is therefore actually-i quite pure.

My invention involves melting the ore', in the caseof molybdenum after it has been at least partially roasted, in order that the molybdenum trioxide or tunsten trioxide 'then already in a free state in the molten mass, or liberated at definite temperatures,

sulphuric acid does not penetrate into the condensing space and consequently the material composing thefurnace, tube vand filter cannot be attacked Y in any way thereby,-

` which might also cause damage to these parts of the apparat-us as well as introduce impurities into the molybdenum trioxide and tungsten trioxide formed. Moreover, all additional impurities con-tained in the ore are lnaturally retained in the melt. It is quite impossible for such impurities, iviz, very small particles ofl the finely divided ore, t be carried along in the manner referred to.

The oxide obtainedby evaporation from the,

melt, condenses from a vapour phase of sub stantially higher density than could ever be produced by sublimation, and the grain of the condensed oxide is therefore completely different :from that of the sublimated oxide.- Oxygen is not employed at all, which not only means a saving in expense, but alo assures that the. processaccording to the' invention will have a satisfactory result. vasv oxygen is to be considered as detrimental.

all other products containing molybdenum i tions prevailing.

In comparison with all known processes of this kind the inventidn entails the further advantage that Waste material from the manufacture of bars and wires, as well as or tungsten, can be added to the ore to b e worked up; they are introduced into the molten mass in such a quantity as would be soluble therein under the particular condi;

A description will now be given of a form of carrying out the process according to the invention, as applied to the production of molybdenum trioxide.

As primary material use is made of partially or completely roasted molybdenite or those molybdenum products which contain such an amount of molybdenum trioxide that upon being heated to temperatures of about mass; the'molybdenum products arc heated in this crucible to temperatures of 1000 C. and over, for example by means of a metal coil vE wound around the' outside of the crucible and traversed by electric current.

'During this heating the crucible is set into` continuous slow rotation. It is mounted upon an inclined axis A which preferably makes an angle of 45 with the horizontal and is Adriven by an electromotor or the like M. It has beenfound that the processcan be carried out all the more successfully the more acute is the angle which the axis'A makes with thehorizontal. It is limited by the fact that naturally `the mass in the crucible must not be allowed to flow out. The effect of this inclined sition of the crucible is to give a large (elliptical) surface to the material which is being melted; the rotation causes the viscous melt to be carried along by the crucible walls and be as it were stirred round, so that fresh parts of the melt are continually coming into contact with the outside air. By this means in the first stage of operation, any sulphide still 'present in the molybdenite is completely oxidized to molybdenum trioxide, and in the second stage the largest possible surface is available for the evaporation.' l

The crucible with its drive is mounted von a movable frame R, which in' this stage of the process is not in the position shown but displaced to the 'right to such an extent that the opening'of the crucible T lies below or in the funnel B, from which a conduit for instance leads into the o en air. In this stage of the process no mo ybdenum trioxide v is yetevaporating or at any rate only in thc very smallest and therefore absolutely neg-V ligible quantities.

With a properly dimensioned crucible a treatment of about 1/2-2 hours will be sufiicient to convert all the molybdenum sulphide present in the ore into molybdenum .trioxide, which will be evident from the fact that no more sulphuricacid vapours escape from the crucible.

The frame R with the crucible is now 4pushed to the left under the hood C, 'into The effect of this current of air is that an ried at a great speed into the chamber 'K,

inwhich owing to a decrease in the speed, it condenses from the vapourized state; owying to the considerable draught in the pipe D it is impossible for condensation to take place there to any appreciable extent. i

In the chamber K the cooled oxide falls like snow on the bottom, Without coming into contact -with any foreign substances.

The amount of oxide which settles immedil ately on the bottom and on the walls of the chamber is naturally so small that any possible impurities in these outside layers are negligible, and even if these outside layers should be conveyed away with the absolutely pure layers lying above them, they cannot have any appreciable effect upon the purity of the oxide. Moreover, it is not necessary to fear any impurities of vthis kind, as no chemical reaction can take place between the cold oxide and the walls of the chamber and there is no sulphuric acid present, and impurities of a merely mechanical nature can be avoided by lining or coating the chamber with sheets of glass.

The oxide collected in the chamber K is accordingly in the perfectly pure state which4 5 has 'been aimed at for so many years.

In addition, its ineness and state of division make it preeminently suitable for the production of metallic powder for the manufacture of wires, even those of finest diameter and more especially for ductile vsheet metal. Although it is 'obtained in a loose sta-te it is nevertheless sufficiently dense to produce metallic powder of very high quality. In special cases it is sufiicient to pass it through agate or steel rollers, exerting a mutual pressure oi about kgs., a densityl lbeing obtained which is not less than that of the oxide hitherto produced exclusively Iby Wet processes. In spite of this the grain of the oxide obtained according to the iuvention is considerably finer than thatA of the oxide produced by Wet processes, which in conjunction with the high purity of the oxide is of very great advantage for the further working up into wires and sheet metal. A further advantage of the fine grain obtained is that when the oxide is reduced to pure metal, the attacking surfaces for the A hydrogen are considerably increased as compared with a coarser grain,l which makes it. possible for the hydrogen on the one hand to be more fully utilized and on the other hand to be passed atl a substantially reduced speed through the stratified oxide. Consequently there is a considerable saving in hy; drogen when the oxide produced according to the invention is further worked up. The inventor'has `found in practice that when .the reducing tube has a dia-meter of 56 mms. and oxide contained in the carrier is continually passed through the tube according .to the counter current principle, a velocity of flow of 1 m. per minute of the hydrogen is suflicient to obtain the finefgrained metal required for further working up into very ine Wires, if a molybdenum trioxide produced according to the `present invention is employed; butpwhen use is made oI the bestmolybdenum trioxide hitherto obtainable on the market it is necessary for Working up the same amount of acid and under other- `wisel equal conditions, to employ a velocity of flow of the hydrogen of 5 8 ms. per minute to obtain a metal suitable for further working up into finest wires, etc. The fineness of the grain in the latter case is, however, considerably less than When the oxide produced according to the linvention is worked up with a fraction of the quantities of hydrogen hitherto required.

A fine pure metallic powder can be ob- ,tained at a low cost according to the invention and can be far more satisfactorily sintered than the finest-grained metallic powder hitherto obtainable, which is nevertheless coarser than that which can be reduced from an oxide produced according to the invention. The iner the metallic powder employed, the, lessis the energy required for producing a sintered bar from the bar obtained by pressure from the powder. Assuming an equal pressure and equal Weight of the pressed bars, the voltage and intensity of current required for the sintering is considerably less the finer-grained is the presed bar. If, for example 55 kilowatt hours electric energy were required for sintering the.

usual type of pressed bars, the same sintering can be carlried'out, When working up an oxide according to the invention, witlr only 22 kilowatt hours electric energy; the resulting sintered bars then possess au equally fine-grained structure. More-over, when sin tering absolutely pure oxide -it is possible to lcmploytemperatures very near to the melting point, whereas in the case of oxides con- Ltaining only a few' hundredths percent impurities, there is no possibility of using Such high temperatures during the sintering, as parts inside the bar may become melted.

The higher the sintering temperature the sooner is the sintering process ended and the greater is the output and utilization of an ordinary sintering plant. .The inventor was able in practice to obtain au increased output of about 66% by this method.

The process according to the invention can be carried out Without any difficulty on the smallest and largest scales, requires no lil) trained stall' and can be carried out with of the labor, which has hitherto been required for the ysame purpose in .an average-sized factory. Also, the size of the Work-rooms required is decreased up to 10% .as compared with those hitherto required.

The process according to the invention can be carried out advantageously at the mine itself. If the orc Were to be conveyed from the mine to Works some distance away the cost of transporting the Weight in crude rock would be equal to that which would have -to be expended for lworking up the roasted molybdenite into oxide at the mine more,'can be used in treating the molybdic compounds as compared with the temperatures used inthe other case, Which as Yalready mentionedV can be carried out at most at 800 C., as otherwise it approaches too near tothe melting point and the molten lmass would flow out of the inclined retorts used in that case which are necessarily open at the lowestpoint. Finally, no addition of oxygen is used -in the present invention, so that therefore the cost .of the oxygen pre,- pared in cylinders or where these are not obtainable, the erection of a special plant for producing o gen, is saved in advance.

The examp e which has just been described can also be applied to the production of tungsten trioxide, W03. Naturally a correspondingly higher melting temperature must then be adopted and, therefore a crucible of 'zirconium oxide, for example, should be used instead of a quartz Crucible.

Fundamentally the invention can be suitably appliedto theproduction of all volatile oxides, which melt after they volatilize. The linvention is especially adapted for use in connection with ores, the evaporat-ing and melting points of which closely approximate each other. l/Vhen'used in connection with ores whose evaporating and melting points differ widely, some additional means well known to those skilled in the art must-be used, such, for example, as controlling the pressure in the container for the ore.

Finally it may be pointed out that naturally it is also possible to carry out'a cont tinuous process if, for instance the frame R is arranged so as to be capable of rotating and two crucibles T provided thereon in 'such a manner that one Crucible is under the suitably increasing the one or other kind of outlet. In that case either the ore lissimultaneously melted in several crucibles or thel oxide simultaneously evaporated from several crucibles. In order to avoid moving the containers (Crucible/s) and in order to prevent in part the provision of two different outlets, the crucible'or the container may also be arranged fixed under one sin le outlet, which is forked at a suitable polnt and on the one hand leads to the condensing chamber and onv the other hand to the open air or any other suitable place, Oand devices, for instance Hap valves may be provided at the point of bifurcation,.which during the heating or melting of the ore lead the resulting vapors into the open air, but while the molybdenum trioxide or tungsten trioxide is evaporating lead these vapors into the condensation chamber. The complete sepay ration ofthe outlets is however decidedly ton be preferred for assuring the absolute purity of the oxide obtained.

The introductory clause of the appended I' claims is intended to indicate that themethod is applicable tov the production of either molybdenum trioxide or tungsten trioxide.

What I claim is l.' In a process for the production of molyb-v denum trioxide and tungsten trioxide from their` ores, melting the respective ore and evaporating the oxide from the ore while in the molten state. 1

2. In aV process for the production of molybdenum trioxide and tungsten trioxide from its ore, roasting and subsequently melting 'the ore and condensing the oxide vapors evaporating from the ore while in the molten state.

3. A process for the production of molybdenum trioxide and tungsten trioxide from" their ores, consisting in melting the ore and los driving oi the oxide vapors by means of a 4. A process for the production of molybdenum trioxide and tungsten trioxide from their ores, consisting in melting the ore and 'withdrawing by suction the oxide vapors evolved from the molten mass.

5. A' process for the roduction of tungsten trioxide and molybdenum trioxide from their ores, consisting in melting the ore ilin one stage and removmg the ,vapors in a subsequent stage, while maintaining the heat of out contacting any heated parts into a con'- the ore. densmg space, and precipitating the oxide '6. A process for the roduction of tung in said space. v

sten trioxide and mo1yb enumtrioxide from In testimony whereof I have signed my 5 their ores, consisting 1n melting the ore, name to this specification.

evaporating the oxide from the molten mass, conveying the evaporated oxide wth- PAUL SQHWABZKOPF. 

